Mattress



FIG. 1 is an isometric view of a front, top, and right side of a first embodiment of an ornamental design for a mattress;

FIG. 2 is an elevational view of the front side of the mattress shown in FIG. 1, a rear being identical to the front;

FIG. 3 is an elevational view of the left side of the mattress shown in FIG. 1, a right side being identical to the left side;

FIG. 4 is a top view of the mattress shown in FIG. 1; and,

FIG. 5 is a bottom view of the mattress shown in FIG. 1.

Any broken lines in the drawings depict environmental subject matter only and form no part of the claimed design. 

CLAIM The ornamental design for a mattress, as shown and described. 